1. Field of the Invention
The present invention relates to methods and apparatus for preparing thin films of Group IBIIIAVIA compound semiconductor films, and more specifically to reacting of Group IBIIIAVIA compound semiconductor films to form photovoltaic devices.
2. Description of the Related Art
Solar cells are photovoltaic (PV) devices that convert sunlight directly into electrical energy. Solar cells can be based on crystalline silicon or thin films of various semiconductor materials that are usually deposited on low-cost substrates, such as glass, plastic, or stainless steel.
Thin film based photovoltaic cells, such as amorphous silicon, cadmium telluride, copper indium diselenide or copper indium gallium diselenide based solar cells offer improved cost advantages by employing deposition techniques widely used in the thin film industry. Group IBIIIAVIA compound photovoltaic cells, including copper indium gallium diselenide (CIGS) based solar cells, have demonstrated the greatest potential for high performance, high efficiency, and low cost thin film PV products.
As illustrated in FIG. 1, a conventional Group IBIIIAVIA compound solar cell 10 can be built on a substrate 11 that can be a sheet of glass, a sheet of metal, an insulating foil or web, or a conductive foil or web. A contact layer 12 such as a molybdenum (Mo) film is deposited on the substrate as the back electrode of the solar cell. An absorber thin film 14 including a material in the family of Cu(In,Ga)(S,Se)2 is formed on the conductive Mo film. The substrate 11 and the contact layer 12 form a base layer 13. Although there are other methods, Cu(In,Ga)(S,Se)2 type compound thin films are typically formed by a two-stage process where the components (components being Cu, In, Ga, Se and S) of the Cu(In,Ga)(S,Se)2 material are first deposited onto the substrate or a contact layer formed on the substrate as an absorber precursor, and are then reacted with S and/or Se in a high temperature annealing process.
After the absorber film 14 is formed, a transparent layer 15, for example, a CdS film, a ZnO film or a CdS/ZnO film-stack, is formed on the absorber film 14. Light enters the solar cell 10 through the transparent layer 15 in the direction of the arrows 16. The preferred electrical type of the absorber film is p-type, and the preferred electrical type of the transparent layer is n-type. However, an n-type absorber and a p-type window layer can also be formed. The above described conventional device structure is called a substrate-type structure. In the substrate-type structure light enters the device from the transparent layer side as shown in FIG. 1. A so called superstrate-type structure can also be formed by depositing a transparent conductive layer on a transparent superstrate, such as glass or transparent polymeric foil, and then depositing the Cu(In,Ga)(S,Se)2 absorber film, and finally forming an ohmic contact to the device by a conductive layer. In the superstrate-type structure light enters the device from the transparent superstrate side.
Contrary to CIGS and amorphous silicon cells, which are fabricated on conductive substrates such as aluminum or stainless steel foils, standard silicon solar cells are not deposited or formed on a protective sheet. Such solar cells are separately manufactured, and the manufactured solar cells are electrically interconnected by a stringing or shingling process to form solar cell circuits. In the stringing or shingling process, the (+) terminal of one cell is typically electrically connected to the (−) terminal of the adjacent solar cell. Circuits may then be packaged in protective packages to form modules. Each module typically includes a plurality of strings of solar cells which are electrically connected to one another.
In a thin film solar cell employing a Group IBIIIAVIA compound absorber, the cell efficiency is a strong function of the molar ratio of IB/IIIA. If there are more than one Group IIIA materials in the composition, the relative amounts or molar ratios of these IIIA elements also affect the properties. For a Cu(In,Ga)(S,Se)2 absorber layer, for example, the efficiency of the device is a function of the molar ratio of Cu/(In+Ga). Furthermore, some of the important parameters of the cell, such as its open circuit voltage, short circuit current and fill factor, vary with the molar ratio of the IIIA elements, i.e. the Ga/(Ga+ In) molar ratio. In general, for good device performance the Cu/(In+Ga) molar ratio is kept at around or below 1.0. On the other hand, as the Ga/(Ga+ In) molar ratio increases, the optical bandgap of the absorber layer increases and therefore the open circuit voltage of the solar cell increases while the short circuit current typically may decrease. It is important for a thin film deposition process to have the capability of controlling both the molar ratio of IB/IIIA, and the molar ratios of the Group IIIA components in the composition.
Two-step deposition techniques, involving deposition of a series of films to form a precursor film stack and then reaction of the precursor film stack to form the compound absorber are often used to fabricate CIGS layers. Individual thicknesses of the films that form the stacked precursor film layer must be well controlled because their thicknesses influence the final stoichiometry or composition of the compound layer after the reaction step. It is known that the structural and electrical properties of CIGS layers are affected when doped with Group IA alkali metals such as sodium (Na), potassium (K) and lithium (Li). Especially, incorporation of very small amounts of Na into CIGS layers has been shown to be beneficial for increasing the conversion efficiencies of solar cells fabricated using such layers. Doping CIGS layers with Na can be achieved by various ways.
One popular method involves Na diffusion from glass substrates. Na diffuses into the CIGS layer from the substrate if the CIGS layer is grown on a Mo-contact layer deposited on a Na-containing soda-lime glass substrate. This is, however, an uncontrolled process and can cause non-uniformities in the CIGS layers depending on how much Na diffuses from the substrate through the Mo-contact layer.
Other popular dopant incorporation methods include forming a CIGS precursor including a thin Na-containing layer, such as a NaF layer, so as to allow Na from the Na-containing layer to diffuse into the growing CIGS absorber during the reaction of CIGS precursor to thereby dope the CIGS layer. However when such Na-containing layers are used for CIGS doping, both the small amounts used, often not more than 2 atomic percent, and the high diffusivity of Na at CIGS reaction temperatures make controlled doping CIGS with Na a challenging process. Accordingly, controlling the Na-containing layer thickness is critical for this doping technique. Studies have shown that if the amount of Na-dopant introduced through the Na-containing layer is inadequate, poor conversion efficiencies are observed in such inadequately doped solar cells. However, if the amount of Na dopant is excessive, some properties of Group IBIIIAVIA compound layers, such as their crystalline properties, mechanical properties and especially their adhesion to their substrate, deteriorate.
As aforementioned, controlled doping of Group IBIIIAVIA compound layers with alkali metals improve their quality in terms of yielding higher efficiency solar cell devices. Considering the shortcomings of presently available doping methods, it is highly desirable to develop new methods of introducing and controlling alkali metals in Group IBIIIAVIA layers.